The invention relates to a concentration network with pulse amplitude modulation (PAM), used in a time division multiplex telephone exchange for connection to a set of four coders of subscriber's lines divided into sub-groups of 2.sup.n lines, 2.sup.p sub-groups being combined into a "group", the total number of groups being 2.sup.q (n, p and q being integers). The total number of subscriber's lines connected to the set of 4 coders by the concentration network according to the invention is therefore 2n+p+q which, by way of example, can be more than 1000 subscriber's lines.
Hitherto it has been possible to concentrate only 512 subscriber's lines in 64 time slots. This restriction is due to the fact that the use of a PAM bus is limited since, when the bus exceeds a certain length substantially proportional to the number of subscriber's lines connected thereto, crosstalk (i.e. echo phenomena and overshooting which is damped too slowly) occurs between each pair of consecutive pulses. These effects become even worse when an increase is made in the number of time slots per frame conveyed by the PAM bus.
The concentration network according to the invention can obviate these disadvantages, since it concentrates the PAM pulses in a first and second bus alternately. In each bus, therefore, there is a wider spacing between each pair of consecutive pulses, so that crosstalk can be reduced and the bus length can be increased. For example, 1024 subscriber's lines can be connected to 4 multiplex lines having 32 time slots, which has hitherto been technologically impossible.